Presently available evidence suggests that after certain types of bone marrow injury, different precursor cells are necessary to reconstitute the supporting connective tissue and microcirculation on the one hand and the blood forming marrow cells on the other. Preliminary studies further indicate, that the differentiation of pluripotential stemcells into erythroid and granulocytic cell lines depends on the supporting non-hemopoietic cells. An experimental model has been developed to test these hypotheses in mice, using the omentum as a transplantation site and using a new chromosome market to establish cell origin in transplants containing more than one cell type. If the concepts proposed can be verified, it would open the way to accelerating regeneration of marrow after certain types of experimental injury and possibly to the treatment of myelofibrosis in man by autologous "cellular engineering". Experimental designs to explore these possibilities are included in the present proposal.